This seems fine but why add these functions when you only have to type two lines to get this result with the current API?

```python
>>> r = p1 * R.x + p2 * R.y + p3 * R.z
>>> v = r.dt(R)
>>> a = v.dt(R)
```

What is the use case where it is a pain to do what I've typed above?

Also, why does ReferenceFrame have a dt method? Is that new? It seems odd to say "Differentiate this frame".